Method of constructing an electronic assembly having an indium thermal couple and an electronic assembly having an indium thermal couple

ABSTRACT

According to one aspect of the invention a method of constructing an electronic assembly is provided. The electronic assembly is constructed from a semiconductor package including a package substrate and a semiconductor chip mounted to the package substrate, a thermally conductive member, and a substance including indium. The method comprises securing the thermally conductive member and the semiconductor package in a selected orientation relative to one another with the thermally conductive member on a side of the semiconductor chip opposing the package substrate and with the substance located between the semiconductor chip and at least a portion of the thermally conductive member. The substance is thermally coupled to the semiconductor chip on one side and thermally coupled to the portion of the thermally conductive member on an opposing side.

BACKGROUND OF THE INVENTION

[0001] 1). Field of the Invention

[0002] This invention relates to a method of constructing an electronicassembly and to an electronic assembly which may be made according tothe method of the invention.

[0003] 2). Discussion of Related Art

[0004] Integrated circuits are formed on semiconductor wafers. Thewafers are then sawed into semiconductor chips. Each semiconductor chipis then mounted to a package substrate. An integrated circuit within thesemiconductor chip can be powered up and data signals can be sent to andreceived from the integrated circuit via the package substrate.

[0005] When the integrated circuit is powered up, heat is generated onthe semiconductor chip which could cause destruction of the integratedcircuit if the heat is not transferred away. A thermally conductiveplate is often located next to the semiconductor chip. A thermallyconductive grease may be located between the semiconductor chip and thethermally conductive plate. The thermally conductive grease contacts thesemiconductor chip and the thermally conductive plate on opposing sidesand acts as a thermal couple between the semiconductor chip and thethermally conductive plate. Heat can then be transferred from thesemiconductor chip through the grease to the thermally conductive plate,from where heat can be transferred to a heat sink or other device and beconvected into the ambient.

[0006] The use of a grease as a thermal couple is often unsuitable forhigh power applications. A thermally conductive grease does not transfera sufficient amount of heat when a large amount of heat is generated ona semiconductor chip. One reason why a thermally conductive grease isnot a good conductor of heat is because there are no metals in athermally conductive grease. Metals, on the other hand, are usually alsoelectrically conductive. The use of a metal as a thermal couple istherefore usually avoided because an electrically conductive metal maycause shorting between components of the semiconductor chip or thepackage substrate.

SUMMARY OF THE INVENTION

[0007] According to one aspect of the invention a method of constructingan electronic assembly is provided. The electronic assembly isconstructed from a semiconductor package including a package substrateand a semiconductor chip mounted to the package substrate, a thermallyconductive member, and a substance including indium. The methodcomprises securing the thermally conductive member and the semiconductorpackage in a selected orientation relative to one another with thethermally conductive member on a side of the semiconductor chip opposingthe package substrate and with the substance located between thesemiconductor chip and at least a portion of the thermally conductivemember. The substance is thermally coupled to the semiconductor chip onone side and thermally coupled to the portion of the thermallyconductive member on an opposing side.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention is further described by way of example withreference to the accompanying drawings wherein:

[0009]FIG. 1 is a sectioned side view of a lid which is used inconstructing an electronic assembly according to an embodiment of theinvention;

[0010]FIG. 2 is a sectioned side view of the lid after a first sheet ofa first alloy is inserted into a recess of the lid;

[0011]FIG. 3 is a sectioned side view after the first sheet is heated tocause melting of the first alloy;

[0012]FIG. 4 is a sectioned side view of the lid after the first alloyis solidified;

[0013]FIG. 5 is a sectioned side view of the lid, further showing asecond sheet that is located within the recess;

[0014]FIG. 6 is a sectioned side view illustrating how the combinationof FIG. 5 is heated and how voids in the first alloy are massaged out;

[0015]FIG. 7 is a sectioned side view of the lid, the first alloy, andthe first sheet after being cooled;

[0016]FIG. 8 is a sectioned side view of components of an electronicassembly including the lid, the first alloy, the second sheet, andfurther including a semiconductor package having a package substrate anda semiconductor chip, and further including a third sheet of a secondalloy;

[0017]FIG. 9 is a sectioned side view of the components of FIG. 8 afterthe second sheet is located on the third sheet;

[0018]FIG. 10 is a sectioned side view of the components of FIG. 9 afterthe first alloy, the second sheet, and the third sheet are heated tocause melting thereof into a mixture; and

[0019]FIG. 11 is a sectioned side view of the components of FIG. 10after cool down and solidification of the mixture.

DETAILED DESCRIPTION OF THE INVENTION

[0020]FIG. 1 to FIG. 11 of the accompanying drawings illustrate a methodof constructing an electronic assembly. An indium containing alloy islocated within a recess within a lid. The indium containing alloy isthen heated which causes undesirable voids to form therein due toreaction between the alloy and the material of the lid. The voids aremassaged out by heating the alloy and directing a jet of air onto thealloy. A sheet of indium, having a higher melting temperature than thealloy, is used to prevent splattering of the alloy when the jet of airimpinges thereon. The cap and the alloy are then assembled together witha semiconductor package in an electronic assembly. The alloy, primarilybecause of the use of indium, has good thermal conductivity. The alloyis an electric insulator which ensures that components of thesemiconductor package are electrically isolated from one another. Thealloy does not break through into the material of a package substrate ofthe semiconductor package. The alloy also has a relatively low meltingtemperature which makes it possible to melt the alloy without causingdestruction of an integrated circuit in a semiconductor chip of thesemiconductor package. Regardless thereof that the alloy causes voidswhen being heated, the voids are massaged away from a heat conductingportion of the alloy.

[0021]FIG. 1 of the accompanying drawings illustrates a thermallyconductive lid 10 which is used in constructing an electronic assemblyaccording to the invention. The lid 10 may be made of a material such ascopper having good thermal conductivity. The lid 10 includes a centralportion 12 and four sidewalls 14. A recess 16 is defined above thecentral portion 12 within the sidewalls 14.

[0022] The lid 10 is cleaned and then, as shown in FIG. 2, a first sheet18 of a first alloy is inserted into the recess 16. The first sheet 18is sized to fit between the sidewalls 14 and almost fills the recess 16.The first alloy preferably includes mass ratios of 44.7% bismuth, 22.6%lead, 19.1% indium, 8.3% tin, and 5.3% cadmium. A sheet of an alloy ofthis kind can be obtained from Indium Corporation of America in Utica,N.Y.. The choice and characteristics of such an alloy will, be evidentfrom the description that follows.

[0023] The lid 10 and the first sheet 18 are then heated to above themelting temperature of about 115° C. of the first alloy to cause meltingof the first alloy 18 as shown in FIG. 3. The indium in the first alloy18 reacts with the copper of the lid 10 to cause bubbles or voids 20within the first alloy 18. Voids within the first alloy 18 areundesirable for heat conduction purposes. The number of voids 20 arehowever much fewer than when pure indium is used instead of the alloy18.

[0024] The first alloy 18 is then allowed to cool to causesolidification thereof as shown in FIG. 4. The voids 20 are then trappedwithin the solidified first alloy 18.

[0025] As shown in FIG. 5, a second sheet 22 is then located on top ofthe second alloy 18. The second sheet 22 is typically about 2 mm thickand is preferably made of pure indium. The second sheet 22 is located upto a rim 24 of the lid 10.

[0026] As shown in FIG. 6, infrared radiation 26 is then used to heatthe lid 10 from below. Heat is transferred from the lid 10 to the firstalloy 18. The combination of the lid 10, the first alloy, and the secondsheet 22 is heated to a temperature which is above the meltingtemperature of 115° C. of the first alloy 18. The substantially pureindium of the second sheet 22 has a melting point of about 135° C. Thecombination of the cap 10, the first alloy 18, and the second sheet 22is heated to a temperature which is below the 135° C. melting point ofthe second sheet 22. The first alloy 18 is thereby melted but the secondsheet 22 remains solid. The lower melting point of the first alloy 18,when compared to the substantially pure indium of the second sheet 22,is primarily due to the inclusion of the lead within the first alloy 18.The lead is preferably present between 0.5% and 30% by mass.

[0027] A nozzle 28 is used to direct a jet of air 30 onto a centralportion of the second sheet 22. The jet of air 30 impinges atsubstantially right angles onto the second sheet 22 and spreads from acentral portion of the second sheet 22 outwardly. Because of deflectionof the jet of air 30, a force is created on the second sheet 22 fromwhere the force transferred to the first alloy 18. The force created bythe air massages the voids 20 outwardly away from a central portion 32of the first alloy 18. Most of the voids 20 escape from between aninterface between the rim 24 and an edge of the second sheet 22. Somevoids 20 may still remain in outer portions of the first alloy 18. Thecentral portion 32 of the first alloy 18 is however substantially orcompletely free of voids. The first alloy 18 is then allowed to cool tocause solidification thereof as shown in FIG. 7. Some voids 20 may belocated in outer regions of the solidified first alloy 18. The secondsheet 22 is located on the solidified second alloy 18.

[0028]FIG. 8 illustrates the cap 10, including the first alloy 18 andthe second sheet 22 which is inverted, and a semiconductor package 34which is also used for constructing an electronic assembly according toan embodiment of the invention. The semiconductor package includes apackage substrate 38 and a semiconductor chip 40.

[0029] The package substrate 38 is at least partially made of andielectric material. The dielectric material may for example be a resinsuch as bismateinite triazine resin which forms a surface thereof. Anarray of solder balls in the form of a ball grid array 42 is located ona lower surface of the package substrate 38.

[0030] The semiconductor chip 40 is typically made of a semiconductormaterial such as silicon and has an integrated circuit (not shown)formed therein. An array of solder bumps 44 are formed on an uppersurface of the semiconductor chip 40 containing the integrated circuitand the semiconductor chip 40 is then flipped as shown in FIG. 8 so thatthe solder bumps 44 are at the bottom according to a process commonlyreferred to as “controlled collapse chip connect” (C4). Thesemiconductor chip 40 is located on an upper surface of the packagesubstrate 38 with the solder bumps 44 located between the packagesubstrate 38 and the semiconductor chip 40. The semiconductor package 34is then heated and allowed to cool, thereby causing attachment of theother bumps 44 to the package substrate 38.

[0031] A third sheet 50 of a second alloy is located over thesemiconductor chip 40. The composition of the second alloy of the thirdsheet 50 may be the same as the composition of the first alloy 18.

[0032] As shown in FIG. 9, a lower surface the second sheet 22 is thenbrought into contact with an upper surface of the third sheet 50 so thatthe combination of the second sheet 22, the first alloy 18, and the lid10 rests on the third sheet 50.

[0033] The combination shown in FIG. 9 is then heated to a temperatureabove 135° C. The first and second alloys 18 and 50 and the second sheet26 melt at a temperature above 135° C. The combination shown in FIG. 9is never heated to a temperature above 150° C., so as to avoiddestruction of an integrated circuit in the semiconductor chip 40. Whenthe first and second alloys 18 and 50 and the second sheet 22 melt, thelid 10 drops onto the package substrate 38 is shown in FIG. 10. Themelted material of the third sheet 50 acts as a wetting layer to ensureproper thermal coupling onto an upper surface of the semiconductor chip40. A melted mixture 52 of the first and second alloys 18 and 50 and theindium of the second sheet 26 fill an entire area between thesemiconductor chip 40 and the central portion 12 of the lid 10. Themelted mixture 52 is therefore in contact with an upper surface of thesemiconductor chip 40 and a lower surface of the central portion 12 ofthe lid 10.

[0034] Because of the choice of the materials of the melted mixture 52,the mixture 52 does not break into the dielectric material of thepackage substrate 38 as may happen if another metal is used instead ofthe mixture 52.

[0035] The combination as shown in FIG. 10 is then allowed to cool tocause solidification of the mixture 52 as shown in FIG. 11. An epoxybead 54 is then located within a filleted interface between the lid 10and the package substrate 38. The epoxy bead 52 secures the lid 10 tothe package substrate 38, thereby finalizing construction of anelectronic assembly 60.

[0036] The mixture 52 provides an efficient thermal coupling between thesemiconductor chip 40 and the lid 10, primarily because of the use of ametal in the form of indium in the mixture 52. The indium in the mixture52 is preferably at least 1% by mass to provide an efficient thermalcouple, although a higher percentage such as at least 10% is a muchbetter thermal conductor. The mixture, including the indium, is anelectric insulator, so that components of the semiconductor package 34,for example the solder bumps 44 or electric traces on the packagesubstrate 38, are electrically isolated from one another even if themixture 52 contacts these components. It should be noted that a centralportion 32, through which heat is conducted from the semiconductor chip40 to the 11d 10, is substantially free of voids, as such ensuring thatthere is an efficient heat conduction path between the semiconductorchip 40 and the lid 10.

[0037] While certain exemplary embodiments have been described and shownin the accompanying drawings, it is to be understood that suchembodiments are merely illustrative and not restrictive of the currentinvention, and that this invention is not restricted to the specificconstructions and arrangements shown and described since modificationsmay occur to those ordinarily skilled in the art.

What is claimed:
 1. A method of constructing an electronic assemblyfrom: (i) a semiconductor package including a package substrate and asemiconductor chip, having an integrated circuit formed thereon, mountedto the package substrate; (ii) a thermally conductive member; and (iii)a substance including indium, the method comprising: securing thethermally conductive member and the semiconductor package in a selectedorientation relative to one another so that at least a portion of themember is located on a side of the semiconductor chip opposing thepackage substrate and the substance is located between the semiconductorchip and the portion of the member, the substance providing a thermalcouple between the semiconductor chip and the portion of the member. 2.A method according to claim 1, wherein the substance contacts thesemiconductor chip on one side of the substance.
 3. A method accordingto claim 2, wherein the substance contacts the portion of the member onan opposing side of the substance.
 4. A method according to claim 1,wherein the substance is a thermally conductive alloy which iselectrically insulative so as to be capable of contacting two parts ofthe semiconductor package without causing a short between the two parts.5. A method according to claim 1, further comprising: heating thesubstance while the substance is in close proximity to the semiconductordie, the substance being heated to a temperature above its meltingpoint.
 6. A method according to claim 5, wherein the substance has amelting point below 150° C. and is heated to above its melting point butbelow a temperature which causes destruction of the integrated circuit.7. A method according to claim 1, wherein the member is a lid having arecess and the substance is a first sheet of material that is located inthe recess, the method further comprising: heating the first sheet inthe recess to cause it to melt.
 8. A method according to claim 7,wherein the lid is of a thermally conductive material that reacts withthe alloy when the alloy is heated to cause it to melt, reaction betweenthe alloy and the material of the lid resulting in voids within thealloy, the method further comprising: removing at least some of thevoids from a portion of the substance, the portion of the substancebeing located between the semiconductor chip and the portion of the lidwhen the lid and the semiconductor package are located in the selectedorientation.
 9. A method according to claim 8, wherein the alloy whenreacting with the material of the lid causes fewer voids than voidscreated by pure indium when reacting with the material of the lid.
 10. Amethod according to claim 8, wherein the thermally conductive materialof the lid is copper.
 11. A method according to claim 8, wherein thevoids are massaged out of the portion of the substance.
 12. A methodaccording to claim 11, wherein the voids are massaged out by a jet offluid that creates a force which moves the voids out of the portion ofthe substance.
 13. A method according to claim 12, further comprising:locating a second sheet of material over the substance; and heating thesubstance to a temperature above its melting point to cause melting ofthe substance but below the melting point of the second sheet ofmaterial so that the second sheet of material remains intact while thesubstance is melted, the jet of fluid impinging on the second sheet andthe second sheet preventing splattering of the melted substance by thejet of fluid.
 14. A method according to claim 13, wherein the materialof the second sheet includes indium.
 15. A method according to claim 13,further comprising: cooling the substance after the voids are massagedout, the substance being cooled to a temperature below its meltingpoint; locating the substance next to the semiconductor chip; andheating the substance to above its melting point to cause reflow of thesubstance over the semiconductor chip.
 16. A method according to claim15, further comprising: locating a third sheet, which includes indium,over the semiconductor chip, the third sheet and the substance beingsimultaneously heated to above their melting points, the third sheetacting as a wetting layer for ensuring proper thermal coupling to thesemiconductor chip.
 17. A method of constructing an electronic assemblyfrom: (i) a semiconductor package including a package substrate and asemiconductor chip, having an integrated circuit formed thereon, mountedto the package substrate; (ii) a lid of a thermally conductive material,the lid having a recess; and (iii) a first sheet of a first alloy, themethod comprising: locating the first sheet in the recess; heating thefirst sheet to above its melting point to cause softening of the firstalloy, the first alloy reacting with the material of the lid to causevoids within the first alloy; locating a second sheet of material on thefirst alloy, the second sheet of material including indium and having ahigher melting point than the first alloy; directing a jet of fluid ontothe second sheet while the second sheet and the first alloy are at atemperature between the melting point of the first alloy and the meltingpoint of the second sheet, the second sheet preventing splattering ofthe first alloy by the jet of fluid, the jet of fluid massaging thevoids out of at least a portion of the first alloy; locating the lid andthe semiconductor package in a selected orientation wherein thesemiconductor chip is next to the recess; and heating the second sheetand the first alloy to a temperature above the melting point of thesecond sheet to cause reflow of the second sheet and the first alloyover the semiconductor chip, the material of the second sheet and thefirst alloy providing a thermal couple between the lid and thesemiconductor chip, with the portion of the first alloy located betweenthe semiconductor chip and the lid, the first alloy being electricallyinsulative so as to be capable of contacting two parts of thesemiconductor package without causing a short between the two parts. 18.The method of claim 17 wherein the alloy includes indium.
 19. Anelectronic assembly comprising: a package substrate; a semiconductorchip mounted to the package; a thermally conductive member located on aside of the semiconductor chip opposing the package substrate; and asubstance, including a metal, between the semiconductor chip and thethermally conductive member, the substance contacting the semiconductorchip on one side and contacting the thermally conductive member on anopposing side so as to thermally couple the semiconductor chip to thethermally conductive member.
 20. An electronic assembly according toclaim 19, wherein the substance is electrically insulative so as to becapable of contacting two parts of the semiconductor package withoutcausing a short between the two parts.
 21. An electronic assemblyaccording to claim 20 wherein the metal is indium.